1. Field of the Invention
The invention relates to an apparatus for electron beam deflection in a cathode ray tube, in particular in a picture tube, which has a deflection coil arrangement for horizontal and vertical deflection of the electron beam and which comprises an auxiliary deflection coil arrangement, by means of which the electron beam can be influenced for the purpose of convergence correction.
2. Description of Related Art
In contrast to conventional television sets, the television picture in projection television sets is produced by means of three monochrome colour picture tubes for the colours red, green and blue, in such a manner that the pictures of the individual colour picture tubes are projected onto a common projection screen. The individual picture tubes are arranged alongside one another, so that their projection axes are not parallel to one another. Furthermore, two of the three projection axes of the picture tubes do not run at right angles to the screen, which leads to projection errors. In addition to these projection errors and independently of them, cushion distortion occurs in the picture tubes, which contributes to poor picture reproduction in the same way as the projection errors.
In order to achieve good picture reproduction, it is thus necessary to correct the images from the individual colour picture tubes. In practice, this is done using auxiliary deflection coils for convergence correction, which are assigned to the individual colour picture tubes. Such auxiliary deflection coils are known in the prior art.
U.S. Pat. No. 4,833,370 discloses an electron beam deflection system for cathode ray tubes which, on the one hand, is equipped with deflection coils for horizontal and vertical beam deflection and, on the other hand, is equipped with auxiliary deflection coils which ensure convergence correction for the beam deflection.
In the case of the known deflection system, the deflection coils and the auxiliary deflection coils are wound on the same core. On the one hand, this results in a particularly compact construction, with the picture tubes having a shortened physical length. On the other hand, the immediate proximity to the main deflection coils causes a large voltage to be induced in the auxiliary deflection coils, and this impedes the required convergence correction. It is therefore necessary to compensate for this induced voltage. This is done by a transformer using which a voltage corresponding to the said induced voltage but of opposite polarity is produced. In the case of this known apparatus, an induced voltage is thus compensated for by a second induced voltage of opposite polarity.
In conventional picture tubes for projection television sets, the main deflection coils and the auxiliary deflection coils are not wound on a common core, but are arranged alongside one another on the neck of the picture tube. As a result of the physical separation, the stray magnetic fields of the main deflection coils, which fields pass through the auxiliary deflection coils, are smaller and, in consequence, the induced voltage caused by them is also smaller. This induced voltage has until now been compensated for by an amplifier circuit which, based on preset correction values, emits to the auxiliary deflection coils the current required for convergence correction. The compensation for the induced voltage caused by the main deflection coils places a considerable load on the amplifier circuit. Furthermore, the associated amplifier is no longer operating in its linear region, which leads to impaired convergence correction.
In the worst case, the induced voltage is so large that the amplifier becomes saturated.